1. Field of the Invention
The present invention relates to a nematic liquid crystal composition. More specifically, the present invention relates to a liquid crystal composition for an active matrix (AM) mode and a liquid crystal display element (LCD) using this liquid crystal composition.
2. Description of the Related Art
A liquid crystal display element of an active matrix mode (AM-LCD) enables highly fine display, and therefore it attracts attentions as the most likely candidate of a liquid crystal display element and is applied to display faceplates for monitors, note type personal computers, digital still cameras, digital video cameras and the like. Characteristics required to a liquid crystal composition for AM-LCD include the following (1) to (5):
(1) In order to expand the temperature range in which the liquid crystal display element can be used, the liquid crystal composition shows a nematic phase in as broad temperature range as possible (the upper limit temperature of the nematic phase is elevated as much as possible, and the lower limit temperature of the nematic phase is lowered as much as possible). PA1 (2) In order to accelerate the response speed of the liquid crystal display element, the liquid crystal composition is reduced in a viscosity as much as possible. PA1 (3) In order to raise the contrast of the liquid crystal display element, the optical anisotropy value (.DELTA.n) of the liquid crystal composition can take a suitable vale according to the cell thickness (d) of the liquid crystal display element. PA1 (4) In order to raise the contrast of the liquid crystal display element, the resistivity value of the liquid crystal composition is elevated, and the voltage-holding ratio of a cell into which the liquid crystal composition is charged is elevated. In particular, the voltage-holding ratio in a high temperature area is elevated. Measurement of the voltage-holding ratio in a high temperature area corresponds to an acceleration test for making sure of the durability of the liquid crystal composition. PA1 (5) In order to miniaturize a battery which is a driving power source for the liquid crystal display element, the liquid crystal composition is reduced in a threshold voltage. PA1 2. A liquid crystal composition according to above item 1, comprising the component A of 5 to 95% by weight and the component B of 5 to 95% by weight, respectively, based on the total weight of the liquid crystal composition. PA1 3. A liquid crystal composition according to above item 1, comprising a component A of 5 to 95% by weight being at least one compound selected from the group of compounds represented by Formulas (1-1) and (1-2) mentioned above, a component B of 5 to 95% by weight being at least one compound selected from the group of compounds represented by Formulas (2-1) and (2-2) mentioned above and a component C of 25% or less by weight being at least one compound selected from the group of compounds represented by the following Formula (3), respectively, based on the total weight of the liquid crystal composition: ##STR2## PA1 4. A liquid crystal display element containing a liquid crystal composition as set forth in any of the above items 1 to 3.
In light of such backgrounds, disclosed in Japanese Patent Application Laid-Open No. 73857/1996 is a liquid crystal composition which has a high voltage-holding ratio and is low in a threshold voltage and which has a suitably large optical anisotropy. Also, disclosed in Japanese Patent Application Laid-Open No. 31460/1997 is a liquid crystal composition which, in particular, has a low threshold voltage, an excellent compatibility at a low temperature and a broad temperature range of a nematic phase while satisfying various characteristics required to a liquid crystal composition for AM-LCD. Further, disclosed in International Publication WO96/11897 are a novel liquid crystal compound which has a large dielectric constant anisotropy and is notably low in a viscosity as a liquid crystal compound for low voltage driving in various modes including an AM mode and a super twist nematic mode (STN mode), and a liquid crystal composition containing the same. Described in Japanese Patent Application Laid-Open No. 251186/1998 is a compound which is similar to the compound of the present invention represented by Formula (1-1) as a compound having a large dielectric anisotropy and a small temperature dependency.
Driving power sources for note type personal computers, digital still cameras, digital video cameras and the like depend on batteries. In order to use these batteries for long time by one charging, a power consumption of LCD has to be reduced. In recent years, these batteries have been miniaturized and come to be elongated in use time by one charging. Accordingly, liquid crystal compositions have come to be desired to be reduced in a threshold voltage while maintaining the characteristics shown in the items (1) to (4) described above.
In order to reduce a threshold voltage of a liquid crystal composition, a liquid crystal compound having a large dielectric constant anisotropy has to be used. If a liquid crystal compound having a large dielectric constant anisotropy is used to prepare a liquid crystal composition, the liquid crystal composition is increased in a viscosity. Accordingly, a liquid crystal display element using a liquid crystal composition having a reduced threshold voltage has been delayed as well in a response speed. As proposed by E. Jakeman et al [Phys. Lett., A, 39 (1972) 69], a response speed is proportional to a square of a cell gap, and therefore a cell gap of a cell constituting a liquid crystal display element can be reduced in order to accelerate the response speed. As shown in the item (3) described above, however, in a first minimum condition of a TN mode, in order to raise the contract, a value shown by a product (.DELTA.n.multidot.d) of a cell gap of a cell constituting a liquid crystal display element and optical anisotropy of a liquid crystal composition is set to a fixed value of about 0.5, and therefore if the cell gap is reduced, the optical anisotropy of the liquid crystal composition has to be inevitably increased.
The composition disclosed in Japanese Patent Application Laid-Open No. 73857/1996 described above has the defects that as shown in the comparative examples in the present invention, while the threshold voltage is low and the optical anisotropy is suitably large, the nematic phase has a too low upper limit temperature and that the voltage-holding ratio in a high temperature area is low and the defects that while the optical anisotropy is suitably large and the nematic phase has a high upper limit temperature, the threshold voltage is too high and the voltage-holding ratio in a high temperature area is low.
The composition disclosed in Japanese Patent Application Laid-Open No. 31460/1997 has the defects that as shown in the comparative examples in the present invention, the optical anisotropy is low and the threshold voltage is high and that the nematic phase has a low upper limit temperature.
The liquid crystal composition for AM-LCD (the composition which does not contain a compound having a cyano group at a terminal of the compound; a compound having a cyano group has a low voltage-holding ratio and therefore can not be used for a liquid crystal composition for AM-LCD) disclosed in International Publication WO96/11897 has the defects that as shown in the comparative examples in the present invention, the threshold voltage is not sufficiently lowered and the optical anisotropy is small.
Compounds having three phenylene rings and one --CF.sub.2 O-- bonding group in a molecule are shown in Examples 32 to 42, 44 and 45 described in Japanese Patent Application Laid-Open No. 251186/1998 but have the defect that the dielectric constant anisotropy (.DELTA..epsilon.) is small, so that the effect of reducing the threshold voltage is small. Further, among the compositions comprising the F base compound having a high holding ratio in the physical property values described in the examples thereof, the compositions having a .DELTA.n of 0.130 or more and a threshold voltage of 1.7 V or less can not be achieved.
As described above, various investigations of liquid crystal compositions have been carried out, and required to a liquid crystal composition for AM-LCD are a reduction in a threshold voltage, which is required for reducing a power consumption while maintaining a high voltage-holding ratio in a high temperature area as maintaining the characteristics shown in the items (1) and (2) described above, and an increase in optical anisotropy, which follows requirement to reduce the gap of the cell in order to accelerate the response speed.